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Apr 1980

Volume 67, Issue S1, pp. S1-S103

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back to top Session CC. Underwater Acoustics V: Radiation and Scattering II
Contributed Papers
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Rough surface backscatter with secondary diffraction and without Kirchhoff approximation (A)

H. Medwin and J. C. Novarini

J. Acoust. Soc. Am. Volume 67, Issue S1, pp. S67-S67 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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The backscatter from a randomly rough, long‐crested surface has been restudied by using the closed‐form Biot‐Tolstoy solution for wedge diffraction [J. Acoust. Soc. Am. 29, 381–391 (1957)] as a building block in the description of the ocean surface. A delta function point source is assumed and the computer adds the facet impulse reflections to the wedge impulse diffractions for the sequence of wedges that closely approximates the surface. This total impulse solution is then transformed to the frequency domain for calculation of backscattering cross section, coherence, etc. The technique permits the identification of the contributions of facet reflection, diffraction, and secondary scatter as functions of sound frequency and angle of incidence. Results are compared with our previous publication [J. Acoust. Soc. Am. 64, 260–268 (1978)] in which only primary diffraction of facets was considered and the Kirchhoff approximation was accepted. [Research supported by ONR.]
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On the frequency dependence of the backscattered field from randomly rough surfaces (A)

P. J. Welton

J. Acoust. Soc. Am. Volume 67, Issue S1, pp. S67-S67 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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In this paper, we investigate the relationship between the physical characteristics of randomly rough surfaces and the surface roughness spectrum (i.e., wavenumber spectrum). This spectrum is important because at low grazing angles (i.e., far from normal incidence), the backscattered field is proportional to the surface roughness spectrum. Consequently, both the frequency dependence and the level of the backscattered field are determined by the surface roughness spectrum. [Part of this work was performed at the University of Paris VII, Paris, France.]
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On the concept of integrated scattering strength and its relation to the theory of scattering by single particles and target strength (A)

Louis C. Maples

J. Acoust. Soc. Am. Volume 67, Issue S1, pp. S67-S67 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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A simple concept of integrated scattering strength is developed from the fundamentals of volume and boundary (surface) scattering and shown to be identical to the target strength of a single scatterer as evolved from the theory of scattering by single particles utilizing the target area or effective cross section of the scatterer as the primary measure. The concept applies equally as well to surfaces such as plates as to solids and volumes, such as rigid or elastic spheres or shells. In order to arrive at a complete specification of integrated scattering strength, particularly for irregular objects, a comprehensive array of statistics is required, from the basic framework of which a simplified. ready‐reference set of numbers may be derived for simple, but meaningful, comparisons of scattering effectiveness.
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Time‐domain response of fluid target in fluid medium (A)

H. Mieras and C. L. Bennett

J. Acoust. Soc. Am. Volume 67, Issue S1, pp. S67-S67 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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The responses of fluid targets are computed using space‐time integral equations formulated in the time domain. The incident pressure wave is a “smoothed‐impulse” with a Gaussian shaped time dependence whose width is of the order of a target dimension. A space‐time integral equation for the pressure field on the outside of the target surface and a space‐time integral equation for the pressure field on the inside of the target surface are solved simultaneously for the pressure and the pressure gradient by stepping on in time and making use of the boundary conditions (continuity of pressure and displacement). The farfield is then computed from these source fields. The technique is applicable to targets of arbitrary contour and is demonstrated for a sphere and right circular cylinder at various angles of incidence. Fluid targets support interior compression waves; sound‐hard and sound‐softargets are treated as limiting cases of this formulation. The technique has been verified for the case of a sphere by comparison with the response computed by classical expansions. [Work supported by NCSC.]
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Discovery of a sunken vessel by parametric sonar (A)

W. L. Konrad and W. L. Clay

J. Acoust. Soc. Am. Volume 67, Issue S1, pp. S67-S67 (1980); (1 page) | Cited 1 time

Online Publication Date: 11 Aug 2005

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During evaluation of a parametric sonar at the Seneca Lake Facility of the Naval Underwater Systems Center, a very strong and discrete sonar return was observed on the mud bottom at a range of 2000 m. Subsequent use of a conventional side‐scan sonar and high‐resolution parametric depth sounder suggests the target is a barge which sank during the latter half of the 19th century. The output of the various sonars used are presented as “A” scan photographs and line scan recordings.
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R‐matrix theory of sound scattering from an absorbing bubble (A)

G. Gaunaurd and H. Überall

J. Acoust. Soc. Am. Volume 67, Issue S1, pp. S68-S68 (1980); (1 page)

Online Publication Date: 11 Aug 2005

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The R‐matrix theory of acoustic scattering, developed by us earlier [G. Gaunaurd and H. Überall, J. Acoust. Soc. Am. Suppl. 1 66, S81 (1979)] is here applied to the case of sound scattering from an absorbing bubble. The mechanical impedance of the scatterer is a meromorphic function of frequency, belonging to the class of Wigner's R functions [E. P. Wigner, Ann. Math. Stat. 53, 36 (1951)] which have poles on the real axis. Developing the S matrix (which contains the R functions) in the one‐level approximation leads to a Breit‐Wigner resonance formula with a frequency shift and a resonance width. The effect of absorption in the bubble consists in a further shift of the resonance frequency and a broadening of the resonance width. Both of these effects (in particular the broadening of the widths) are suitable means for determining the absorption, which is crucial for the echo reduction of a target. [H. Überall is also at Catholic University, Washington, DC 20064, additionally supported by Code 421 of ONR.]
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